1. Field of the Invention
The present invention relates to plasma discharges, and, in particular, to moderate- to high-pressure non-equilibrium plasma discharges for materials processing applications.
2. Description of the Related Art
Plasma surface modification of large web or film substrates using a cold, non-equilibrium plasma has traditionally been accomplished using corona treatments at atmospheric pressure or as a batch process in low-pressure plasma reactors. Corona treaters are frequently used to activate the surface of a polymer for printing or laminating, and predominantly use atmospheric air as the working gas. Corona treaters are usually configured as a dielectric barrier discharge and excited using a low-frequency signal in the frequency range of 60 Hz to 30 kHz. Because of the chemistry restricted by the use of air and the nature of the plasma generated, corona treatment is rather limited compared to low-pressure treatments. Low-pressure treatments can employ a variety of gas chemistries and discharge techniques to obtain a wide range of surface modifications and therefore enhance the utility of a polymer substrate.
Low-pressure microwave discharges, in particular, have been shown to be more efficient at producing ion-electron pairs than lower frequency discharges. For a given input power, a microwave discharge operating in the gigahertz frequency range is capable of generating a higher number density of ion-electron pairs and excited-state species than is a low-frequency RF discharge operating in the megahertz range or the dielectric barrier discharge operating in the kilohertz frequency range. A microwave discharge using common gases such as nitrogen or oxygen an lead to significant surface functionalization of polymer surfaces.
Microwave discharge techniques however are typically restricted to low-pressure operation. A variety of low-pressure microwave discharge techniques have been employed for many years in the processing of semiconductor substrates. These techniques usually operate at pressures of one Torr or less and generate plasmas capable of treating substrates of only modest surface area. For the in-line processing of wide substrates (1.gtoreq.0.2 meter), the requirement of a low-pressure (P.ltoreq.1 Torr) discharge is difficult to obtain and expensive due to vacuum pumping costs. The high efficiency of a microwave discharge, and its ability to effectively functionalize a polymer surface, make microwave discharges desirable for the in-line treatment of various webs and films.
High-power microwaves have been used to produce high-pressure, non-equilibrium, volume discharges by focusing the microwave energy into a region of intense energy density. The critical power flux S.sub.crit for this type of discharge is extremely high (e.g., from about 0.5 to about 2 MW/cm.sup.2, where MW is megawatts). Since microwaves can only be focused to a minimum area on the order of 4.lambda..sup.2, where .lambda. is the wavelength of the radiation, the microwave source must provide a power level P.sub.crit that scales as the square of the microwave .lambda.. Thus: EQU P.sub.crit =(2.fwdarw.8).lambda..sup.2 MW,
where .lambda. is in centimeters and MW is megawatts. Hence, for the commercial industrial microwave frequencies of 0.915 GHz (.lambda.=32.8 cm) and 2.45 GHz (.lambda.=11.8 cm), the required power densities become unattractive for commercial surface processing applications.
As an alternative to focusing microwave energy in free space, wires or dielectric structures can be used to couple microwave energy in a way suitable to generate a high- or moderate-pressure discharge. A wire "broom" has been used to concentrate 0.36-centimeter microwave energy and produce a non-equilibrium discharge in air.
U.S. Pat. No. 3,814,983 discloses a slow-wave structure using a dielectric structure for the generation of a moderate- to low-pressure microwave discharge. The slow-wave structure is rather complex, and relies on an array of dielectric rods to distribute microwave energy over a large volume. Staggered pairs of dielectric rods are spaced at half-wavelength intervals to couple microwaves from a waveguide and then radiate the coupled energy into a working volume. The half-wavelength spacing of the dielectric rods necessitates single-frequency operation. In order to provide more uniform treatment of a substrate, the dielectric array is physically displaced from the substrate.
U.S. Pat. No. 4,955,035 utilizes a variable-dimension waveguide to generate a high-pressure gas discharge laser. The structure does rely on traveling microwaves and variable-waveguide dimensions to generate a non-equilibrium uniform discharge in a laser volume. The plasma discharge is generated directly by using the electric fields of the traveling wave within the waveguide.
U.S. Pat. No. 5,517,085 discloses a method for generating a moderate- to high-pressure microwave discharge by using an annular waveguide resonator. A waveguide is configured as a closed ring with wall slots cut on the interior wall of the ring. Microwave energy is coupled from an external source into the resonant structure by means of a feed probe. The plasma discharge is generated within the internal volume of the ring via power leaking from the wall slots. The plasma working gas are typically separated from the resonator using a dielectric cylinder. The cylindrical shape of this structure, however, does not lend itself well to the treatment of wide substrates, such as films and webs. Also, the structure is not a traveling-wave structure, but rather a resonant structure.
Slotted waveguides have been used for heating applications and as radiators for antenna structures. Such a slotted waveguide structure has been used for the heating of dielectrics using high-power microwaves. This structure employed multiple slots with the slots typically 0.4.lambda..sub.0 in length, where .lambda..sub.0 is the free-space wavelength. U.S. Pat. No. 4,334,229 presents a slotted waveguide structure for use as a far-field antenna. This structure utilizes variable waveguide dimensions and slot location to provide a specified radiation pattern. Dielectrics are incorporated with the waveguide for allowing higher power operation, and the weather-proofing of the structure. Both of these structures were neither designed nor intended for the generation of a plasma discharge. Any generation of such a plasma would diminish the utility of these structures as radiators of microwave energy.